Four cases of Kaposi's sarcoma (KS) are described; one each was associated with Hodgkin's disease, a thymoma, an atypical lymphoid proliferation resembling Castleman's disease, and a cutaneous reticulosis. The association of KS with lymphoproliferative, hematologic, and other neoplasms is thoroughly reviewed, with special attention to tissue diagnosis, relative order of development, and relationship to immunosuppressive treatment. Review of 65 accepted cases, including ours, showed that 85% of patients with KS and a known order of development appeared with KS either simultaneously or after the second neoplasm. There was a disproportionate tendency for hematologic malignancy to develop simultaneously with KS. Of the 22 patients who had KS subsequent to a second neoplasm, 18 (82%) had received some form of interval immunosuppression. This evidence and observations in renal allograft recipients, patients with pemphigus, and patients with other autoimmune disorders point to the frequency of a common background of deranged immune status in patients with KS. Therefore, either naturally occurring or iatrogenic immune deficiency likely predisposes patients to KS. Some possible mechanisms for this phenomenon are reviewed.